1. Field of the Invention
The present invention relates to a sheet feeding apparatus which feeds sheets from sheet accommodating portions such as storage units, and an image forming apparatus such as a printer or copying machine having this sheet feeding apparatus.
2. Description of the Related Art
Conventionally, a sheet supplying apparatus which detects overlapping of sheets using detection data from an ultrasonic sensor for one measurement area selected from a plurality of measurement areas based on information acquired from a sheet length recognizing portion to detect multifeeding of sheets has been proposed for image forming apparatuses such as copying machines (see Japanese Patent Laid-Open No. 2005-162423).
Further, to prevent detection error due to the type of sheets (type of paper), temperature and humidity, there has been proposed a feeding apparatus which temporarily stops a sheet which is being conveyed, emits from an ultrasonic wave emitting portion an ultrasonic wave for detecting multifeeding, and detects multifeeding using an output of a ultrasonic receiving portion which receives the emitted ultrasonic wave (see Japanese Patent Laid-Open No. 2006-312527).
According to conventional multifeeding detecting methods, overlapping of sheets is detected based on detection data of one measurement area selected from a plurality of measurement areas, so that, when sheets overlap in the one selected measurement area, it is possible to detect multifeeding of sheets. However, in case of occurrence of unseparated multifeeding that a subsequent sheet overlaps a preceding sheet in an unselected area, there arises a problem that unseparated multifeeding cannot be detected as multifeeding.
Further, with the configuration of detecting multifeeding by temporarily stopping sheets which are being conveyed, it is possible to stably detect the multifeeding state in a portion where the ultrasonic wave for detecting multifeeding of temporarily stopped sheets transmits. However, when sheets are multifed in the other portions, it is not possible to determine multifeeding.